Select NULL values from SQL Server table

Question

I am working with ASP.NET MVC 4 using C# and SQL Server

I am selecting a row of data from the following table

CREATE TABLE [dbo].[Mem_Basic] (
[Id]          INT           IDENTITY (1, 1) NOT NULL,
[Mem_NA]      VARCHAR (100) NOT NULL,
[Mem_Occ]     VARCHAR (200) NOT NULL,
[Mem_Role]    VARCHAR (200) NOT NULL,
[Mem_Email]   VARCHAR (50)  NULL,
[Mem_MPh]     VARCHAR (15)  NULL,
[Mem_DOB]     DATE          NULL,
[Mem_BGr]     NCHAR (10)    NULL,
[Mem_WAnn]    DATE          NULL,
[Mem_Spouse]  VARCHAR (75)  NULL,
PRIMARY KEY CLUSTERED ([Id] ASC)
);

using the following code

public MemberBasicData GetMemberProfile(int id)
{
        MemberBasicData mb = new MemberBasicData();
        using (SqlConnection con = new SqlConnection(Config.ConnectionString))
        {
            using (SqlCommand cmd = new SqlCommand("SELECT * FROM Mem_Basic WHERE Id="+id+"", con))
            {
                try
                {
                    con.Open();
                    SqlDataReader reader = cmd.ExecuteReader();
                   if(reader.Read()==true)
                    {
                        mb.Id = (int)reader["Id"];
                        mb.Mem_NA = (string)reader["Mem_NA"];
                        mb.Mem_Occ = (string)reader["Mem_Occ"];
                        mb.Mem_Role = (string)reader["Mem_Role"];
                        mb.Mem_Email = (string)reader["Mem_Email"];
                        mb.Mem_MPh = (string)reader["Mem_MPh"];
                        mb.Mem_DOB = (Convert.ToDateTime(reader["Mem_DOB"]));
                        mb.Mem_BGr = (string)reader["Mem_BGr"];
                        mb.Mem_WAnn = (Convert.ToDateTime(reader["Mem_WAnn"]));
                        mb.Mem_Spouse = (string)reader["Mem_Spouse"];
                   }
                }
                catch (Exception e) { throw e; }
                finally { if (con.State == System.Data.ConnectionState.Open) con.Close(); }
            } 
        }
        return mb;
    }

This shows the error

Unable to cast object of type 'System.DBNull' to type 'System.String'.

( Mem_Email , MPh .. etc sometimes contain a NULL value.. if the value is null I want return null). Anybody please help me.

Answer 1

只要做一些简短的if，你应该为所有其他变量做同样的事情：

 mb.Mem_Email = reader["Mem_Email"] == System.DBNull.Value ? null : (string) reader["Mem_Email"];

Answer 2

You could save yourself a serious amount of pain here with a tool like dapper ( http://www.nuget.org/packages/Dapper ):

public MemberBasicData GetMemberProfile(int id)
{
    using (var con = new SqlConnection(Config.ConnectionString))
    {
        return con.Query<MemberBasicData>(
            "SELECT * FROM Mem_Basic WHERE Id=@id",
            new { id } // full parameterization, done the easy way
        ).FirstOrDefault();
    }
}

things this does:

• does correct parameterization (for both performance and safety), but without any inconvenience
• does all the materialization, handling nulls (both in parameters and columns) for you
• is insanely optimized (basically, it is measurably the same speed as writing all that code yourself, except fewer things to get wrong)

Answer 3

Alternatively to King King's answer you can write code like this:

mb.Mem_Email = reader["Mem_Email"] as string;

For value types, if the column allows nulls, it's a good practice to map them to nullable value types in C# so that this code reader["Mem_DOB"] as DateTime? works

Answer 4

Change for all columns, that might be NULL from this

mb.Mem_NA = (string)reader["Mem_NA"];

to that

mb.Mem_NA = reader["Mem_NA"].ToString();

Answer 5

Treat the nullable fields:

mb.Mem_Email = System.DBNull.Value.Equals(reader["Mem_Email"])?"":
                                      (string)reader["Mem_Email"];

Do the same for: mb.Mem_MPh, mb.Mem_BGr and mb.Mem_Spouse.

Answer 6

I don't mean to sound like a SQL bigot (which of course means I DO mean to sound like a SQL bigot), but if you followed SQL best practices and used a column list instead of SELECT * you could resolve this problem by using COALESCE on the nullable columns thus:

SELECT
  [Id],
  [Mem_NA],
  [Mem_Occ],
  [Mem_Role],
  COALESCE( [Mem_Email], '' ) AS [Mem_Email],
  COALESCE( [Mem_MPh], '' ) AS [Mem_MPh],
  COALESCE( [Mem_DOB], CAST( '1753-1-1' AS DATE ) ) AS [Mem_DOB],
  COALESCE( [Mem_BGr, '' ) AS [Mem_BGr],
  COALESCE( [Mem_WAnn], CAST( '1753-1-1' AS DATE ) ) AS [Mem_WAnn],
  COALESCE( [Mem_Spouse], '' ) AS [Mem_Spouse]
FROM 
  [dbo].[Mem_Basic];

Your c# code can now dependably process the result set without having to account for outliers (the exception being the dates; you should probably check for whatever default you use in the COALESCE for those (I used the minimum allowable value for a SQL Date variable in the above example), and handle them appropriately.

Additionally, you can get rid of the finally block in your c# code. You wrapped the connection in a "using" block; it will automatically close the connection when you go out of scope (that is the purpose of the "using" block).